Ferromagnetic materials having a rectangular hysteresis cycle



April 24, 1962 A. PIERROT ETAL 3,031,405

FERROMAGNETIC MATERIALS HAVING A RECTANGULAR HYSTERESIS CYCLE Filed Deo. 9, 1957 '7 Sheets-Sheet 1 +B 5m 5pm y. l 07 s A 4 A Awmvmvmunvgx mvmynmmvvun;

AVAVAVAVAVAYAZAVAVAVAVAVA Numana@tnvAvAvAvAvA AVAVAVMQVAVM VVAVAVAVA AvAvAvAvgAvAvAzAvV/f/ muuu AVAVAVVNAVNAVA@avAvAvmvAvA mwunnmvmwAvAvAvAnyAvA mmvAvAvAvAvAvAv);vAvAvAvAvAnzv mi vvznvvvv vvvv Attorney April 24, 1962 A. PIERROT ETAL 3,031,405

FERROMAGNETIC MATERIALS HAVING A RECTANGULAR HYSTERESIS CYCLE Filed Dec. 9, 1957 '7 Sheets-Sheet 2 5m Fig. 3.

m 4 "0 5% n0 /070 200 5%2/2 0X3; 05

40 K 46 50A #e203 5m F/y. 5. 4000 l Z/Zj 0 Inventar Afpevro Ybescroe ByQ/u hf. my(

ttarney April 24, 1962 FERROMAGNETIC MATERIALS HAVING A RECTANGULAR HYSTERESIS CYCLE A. PIERROT ETAL Filed Dec. 9, 1957 7 Sheets-Sheet 3 Ff' .6. 0 Fm g 0 fj/2e F/g. Z rf" Fig. m

l /i Q5 H 00 2.5 5.0 z5 /0 7 Inventor A ltorney April 24, 1962 A. PlERRoT LE'rAl. 3,031,405

FERROMAGNETIC MATERIALS HAVING A RECTANGULAR HYSTERESIS CYCLE Filed Deo. 9, 195'? A'7 Sheets-Sheet 4 Inventor Afehrot Y. Lescmael BWM Attorney April v24, 1962 A. PIERROT ETAL 3,031,405

FERROMAGNETIC MATERIALS HAVING A RECTANGULAR HYsTEREsIs CYCLE Filed Dec. 9, 1957 7 Sheets-Sheet 5 0 ff /0 -8 --6 -4 -2 0+? #4 +6 f8 +10.

80M dw@ A ttorn e y April 24, 1962 A. PIERROT ETAL 3,031,405

FERROMAGNETIC MATERxALs HAVING A RECTANGULAR HYsTEREsIs CYCLE Filed Dec. 9, 1957 'r sheets-sheet e Inventor A-Der roi'. Y- Lescroel ISM/M245@ A Harney April 24, 1962 A. PIERROT ETAL 3,031,405

FERROMAGNETIC MATERIALS HAVING A RECTANGULAR HYsTEREsIs CYCLE Filed Dec. 9, 1957 7 Sheets-Sheet '7 FIG. I7. FIG. I8.

A ttorney United States Patent O FERROMAGNETIC MATERIALS HAVING A RECTANGULAR HYSTERESIS CYCLE Andr Pierrot and Yves Lescroel, Conflans Sainte- Honorine, France, assignors to Lignes Telegraphlques et Telcphoniques, Paris, France, a French company Filed Dee. 9, 1957, Ser. No. 701,612 Claims priority, application France Dec. 14, 1956 3 Claims. (Cl. 252-625) The present invention relates to ferromagnetic materials of the ferrite type, having substantially rectangular hysteresis cycles and methods of manufacturing such materials. Materials of this kind can be employed in magnetic recording devices known as memory devices, magnetic control members, magnetic amplifiers, etc. In such applications these materials are used in the form of cores, usually of toroidal shape or at least of closed shape, without air-gap.

Materials with a hysteresis cycle of rectangular form were already known, particularly alloys of iron or of iron and silicon, of which the magnetic properties are most frequently rendered anisotropic either by cold rolling or by heat treatment under a magnetising field. These materials, generally speaking, have high magnetic moments at saturation and low coercive fields.

The great drawback of these metallic materials, despite their usually high magnetic moment .at saturation, is the low value of their resistivity, which leads to considerable eddy current losses. These high losses result in an increase of the response time and a deformation of the hysteresis cycle, which then loses its character ofk rectangularity as soon as the frequency increases.y yIfit be desired to employ these cores at frequencies of several megacycles per second, they must be obtained in very thin form, of the order of a few microns, and their price immediately becomes prohibitive.

Before explaining the present invention, some notations and definitions are given of the magnetic magnitudes which will be employed in the following, and it is specified that the values of the magnetic moment in gauss, is the product by 41|- of the value of the magnetic moment in electromagnetic c.g.s. units.

A substantially rectangular hysteresiscycle, plotted for a magnetic field practically reaching saturation, is defined by the following coefficients: Y Is=moment at saturation, gausses,

I r=remanent moment corresponding to the cycle at saturation, in gausses,

Ho=coereive field corresponding to the cycle at saturation, in oersteds,

Bdm=nal value of the induction When the magnetising field is made to pass from a value H,n comprised between Hom and 2Hm to the value ;Ll Hom=coercive field, in oersted,

m=% or "coeflcient of rectangularity,

Rm=% or ratio of reetangularity,

l 3,031,405 Patented Apr. 24, 1962 ,ice

.53 It is .also possible, in certain cases, to evaluate the slopes of the substantially vertical and horizontal sides of the hysteresis cycle.`

The quantities:

AI 13h-(xn and AI A Y 14m in which AI and AH are small variations of the moment and of the magnetising field in the vicinity of a given point, and are respectively defined in the vicinity of the ,intersection of the curve representing the hysteresis cycle with the axes of coordinates, Ph corresponding to a zero field and Pv to a zero moment.

For an ideal rectangular cycle, the values of Ph and Pv would tend respectively towards zero and towards infinity.

The permeability a is defined as the initial permeability in the demagnetised state.

The coeliicients of magnetic losses employed are the Jordan coefficients; in only considering the contribution of the eddy current losses to the loss resistance Rp, therefore, the coefficient of eddy current losses is defined by the I ordan relation:

Ri 800 2 F- (T) Rp being the loss resistance, expressed in ohms, f being the frequency, expressed in cycles per second, L being the inductance, expressed in henrys.

The coefficient of eddy current losses Fn, expressed vin ohms per henry, for the frequency f=800 c./s., is measured between 100 and 200 kc./s., for a field of 2 millioersteds and at a temperatureof about 20 C., for magnetic circuits of which the cross-section is about 0.3 cm.

The Curie point 0c, in the following will be, by convention, the temperature expressed in degrees centigrade, above which the material is no longer' ferromagnetic.l

The magnetostrictive effectscan be characterised by the value of the coefficient of magnetostriction at saturation xs, obtained by extrapolating for the demagnetised state the relative curve of variation inthe direction of the applied field, of the length l of the Y by a coefficient of variation of induction in percent per degree:

Bm being the value of thev induction at 20 C.

ABm being the variation of induction between 20 and At'being the corresponding difference of temperature, that 1s to say, in the present case At=60 C.

The induction B1,r1 corresponds to a particular field Hm which is clearly higher than Hc. t The response time is defined by considering two windings with negligible time constants placed on a core made of the magnetic material concerned, this core is 3 l subjected to the magnetising field Hm comprised between H,m and ZHcm, then to the field Anegative impulse I; two positive impulses value l/2; a negative impulse of- I.v The interval between the pulses is 50 microseconds, the rise time of each impulse being 0.3 microsecond. v `The response given in winding N2 to the second impulse is then considered. The response timeV 1- is taken as the time, in microseconds, required forthe voltage produced in winding N2, starting from one tenth of the maximum to pass through the maximum andY return to of the value of this maximum.

The object of the invention is to produce magnetic materials of the ferrite type having on the one hand, substantially rectangular hysteresis cycles, and on the other hand high resistivities p at leastequal to 103 ohms-centi# metre.

The -rectangularity of the cycle is obtained by starting` y By this meansa cycle of rectangularity having a coeiicient of rectangularity m at least equal to 0.8 may be obtained. s

Further according to the invention magnetic materials o f the above kind in addition to the natural stresses obtained during heat treatment may be subjected in the form of the finished core to external artificial mechanical stresses. It has been vertiiied that in this way the rec tangularity of the cycle is further improved; a coefficient of rectangularity ,8m atleast equal to 0.95 is obtained, and, what is particularly remarkable, a rectangularity ratio Rm greater than 0.9 is obtained, a value unequalled hitherto. j

The said external artificial mechanical stresses can be realised by known'p'rocesses, hydraulic, the core being placed in a receptacle into which oil or water is forced under pressure, or mechanical,

(a) The core is surrounded by a band of rubber or v f (c) The wound core is embedded in a resin which,

after compression or preferably after polymerisation, ex- Y e'rcises a considerable stress on cooling.

4, K l of induction as a function of the temperature aB at most equal to 0.5 and Curie points 6c higher than 200 C.

The ferromagnetic materials which are the object of the present invention are ferromagnetic materials of the ferrite type, with substantially rectangular hysteresis cycle, manufactured by compressing a homogeneous mixture of fine powders of metallic oxides under a pressure of from 0.5 to l5 tons per square centimetre and by sub-V jecting the cores obtained from the mixture to a heat treatment consisting of heating to a temperature coma prised between 900 C. to 1,300 C. in oxygen, followed if by cooling for about l5 hours, the said mixture consiste."

ing of ferrie oxide and optionally oxides of trivalent 4 metals ofthe group comprising aluminum and chromium, oxides of at least two bivalent metals of the group containing nickel and copper, and optionally oxides of bivalent metals of the group comprising zinc and cadmium,

characterised in this, that in the said mixture the sum of 'i the molecular percentages of the oxides of the trivalent metals is comprised between 47 and 50, the sum of the molecular percentages of the oxides of the trivalent metals, other than iron, being less than or equal to 2, the sum of the molecular percentages, of the oxides of the bivalent metals of the group comprising nickel and copper being comprised between` 25 and 53; kthe percentage of the copper oxide being comprised bef tween 3 and 15 and the sum of the molecular percentages of the oxides of bivalentmetals `of the group` comprising zinc and cadmium being .at most equal to 25. i

The invention will be described in more detail in the following for ferrites of which the starting compositions satisfy the formula: i

xFe2o3, m0, vouo, szno, yrzo,

nate the molecular percentages satisfying the following relations:

In the whole of the rest of the description the come positions indicated are the starting compositions before f i grinding. The increase of the ironcontent due to the wear of the grinder, being for an averagemill, about 0.8

v molecule of Fe203 percent molecules of ground material;

All forms of cores presenting a closed magnetic cir- Y cuit without an air gap. may be thus treated.

This artificial mechanical stress is applied to the core after heat treatment and the applied stress is maintained during use.

In view of theirv high resistivity, these materials haveV negligible eddy-current losses which enable them to be used at high frequency with low response time (flO microseconds).

The materials in accordance with the invention have coliicients of rectangularity m abd ratios of rectangularity at least equal .to the values indicated above. In addition Y they have inductions at saturation (s) ofthe order of 1,500 to 4,500 gausses` at `20 C. coefficients of variation the given percentages of Fe203 after grinding must be increased by this quantity; it wouldbe necessary to make corrections if a mill were used which wore out more slowly or more quickly. Y

It is well-known that the magnetostriction of a mixed ferrite material depends upon the magnetostriction of each of the ferrites'of which it is composed. Of all the ferrites, only the ferrite of 'iron or magnetic oxide of iron FeO.Fe2O3 (that is Fe304) shows av positive coefficient of magnetostriction.

In accordance with the invention it is necessary in order to obtain a material with substantially rectangular hysteresis cycle to form a ferrite having littlevor no bivalent iron. Now the maximum molecular percentage of the iron oxide being equal to 50 there will only be very little bivalent iron or even none at lall and theconditions required for the material to have a negative coeflicient of magnetostrietion are, therefore, well satised.

The invention will be described in more detail inthe following with reference to the attached drawings:

FIG. 1 represents a substantially rectangular hysteresis i cycle:

l positions of materials in accordance with the invention.

FIG. 3 represents curves of variation of induction Bm, as a function of the molecular percentage of Fe203;

FIG. 4 represents curves of variation of the coefficient of rectangularity m as a function of the molecular percentage of Fe203;

FIG. 5 represents curves of variation of induction Bm, as a function of the molecular percentage of A1203 or of C1203 in a defined compound; 5

FIG. 6 represents curves of variation of the coefficient of rectangularity 18m as a function of molecular percentage of A1203 or Cr203, in a defined compound.

FIG. 7 represents the variation of the induction Bm as a function of Hm for a defined compound;

FIG. 8 represents the variation of Rm, and of ,Bm as a function of H,nn for a defined compound;

FIG. 9 represents the variation of the induction Bm as a function of the temperature for two defined compounds; y

FIG. 10 represents the variations of the coercive eld Hem, as a function of the temperature for two defined compounds;

FIG. l1 represents the Variation of the ratio of rectangularity m, as a function of the temperature for two defined compounds;

FIGS. 12 to 16 represent hysteresis cycles of material according to the invention.

FIGS. 17, 17a and 18, 18a each represent, in horizontal and Vertical section a core enveloped in a polyester resin;

FIG. 19 represents, for different fields, hysteresis cycles for a magnetic core not enveloped and for the same core enveloped in polyester resin.

In FIG. l which represents a rectangular hysteresis cycle corresponding to a eld Hm, are indicated the induction Bm=OR, the remanent induction B,m=0P, the induction Bdm=0S corresponding to the field amis RVB-FUR and, for the ratio Km:

FIG. 2 represents a triangular diagram corresponding to a material according to the invention, of which the three components are: the sum of the molecular percentages of the oxides of trivalent metals Fe203, Cr203, A1203, the sum of the molecular percentages of the oxides of bivalent metals NiO and CuO and the sum of the molecular `percentages of bivalent metals ZnO and CdO. The figurative point of the composition with rectangular hysteresis cycle, in accordance with the invention, must be located inside the hatched zone A, `B, C, D, which appears in the formof an elongated parallelogram. Above the limit C -B there may again be compositions satisfying `the condition lof rectangularity mOO but the Curie point 6., becomes lower as the molecular content of Zn0 increases and the condition oB 0.50 can no longer be guaranteed.

In all the following, the characteristics of the hysteresis cycle have been marked on the linx-meter on toroidal cores of the approximate dimensions:

External diameter=34-3 mm., Internal diameter=27.3 mm., and Height-:1l mm. v

fi wound with 100 turns of copper wire 0.30mm. in diameter for the field winding and 200turns of copper wire 0.15 mm. in diameter for the induction windings.

FIG. 3 shows curves of variation of induction Bm, marked at Hm=l0 ocrsteds as a function of the molecular percentage of Fe203. In all these compounds, the molecular percentage of Cu0 wasaS, and each curve corresponds to a family of compounds of constant molecular content of Zn0; these molecular percentages of ZnO corresponding to the four curves are respectively 0, 5, l0 and 15. All these compounds have been treated under the same conditions; in particular they have been sintered at 1200 C. for two hours in oxygen. It will be noted that when the molecular percentage of Z110 increases, the maximum of Bm is shifted to the right, that is to say, towards higher molecular percentages of Fe202 and that this maximum reaches a higher value.

In FIG. 4, curves of variation of the coefficient of rectangularity m, for Hm=l0 ocrsteds are shown as'a function of the molecular percentage of Fe203. All the compounds compared are the same as those of which the properties are represented in FIG. 3 and they are grouped in the same manner. It will be noted that the maximum of m occurs approximately, for the same molecular percentage of Fe203 as that which gives the maximum of m. The value of the maximum of m is adjacent to 0.90.

FIG. 5 shows the variation of the induction Bm, marked at 10 oersteds, for a family of compounds:

(4Q-y) rezos, yrzos, ssNio, sono, iozno' as a function of y; the curve C1 is relative to the case in which T represents aluminum and the curve C2 is relative to the case in which T represents chromium. It is clear that the substitution of molecules of A1203 or of Cr203 for an equal number of Fe203 molecules' in the compound:

l49Fe203, 36Ni0, 5CuO, lOZnO has the effect of reducing the maximum induction.

In FIG. 6 is represented the variation of the coefiicient of rectangularity ,6m marked at l0 oersteds, for the same family of compounds as that corresponding to FIG. 5, as a function of the molecular percentage of A1203 (curve C3) or as a function of the molecular percentage of Cr203 (curve C4). All these compounds have been treated under the same conditions and in particular have been sintered at 1,200 C. for two hours in oxygen. The coefificient for rectangularity m decreases slightly when the molecular content of Cr203 or of A1203 increases. It will be seen moreover, that in both cases `the influence of Cr203 is more marked than that of A1203.

It is possible with reference to FIGS. 7 and 8 toexamine the influence of the work field HIn on the characteristics; the results are represented which are obtained on the ferrite composition:

which has been sintered at 1,200 C. for two hours in oxygen.

FIG. 7 represents the curve of variation of the induction Bm, as a function of the field Hm; it is in fact the curve of first magnetisation. Y

FIG. 8 represents the curves of variation of the coefficient of rectangularity m and of the ratio of rectangularity Rm as a function of the field Hm. It will be seen that ,6m increases very rapidly at the start, passes through a slight maximum of the order of 0.93 at 5 oersteds, then decreases very slowly. `The curve of Rlm on the other hand, shows a very clear maximum for Hm=3.4 ocrsteds of which value Rm=0.745; the part situated on the right of the maximum is very rapidly decreasing.

The curves of FIG. 8, plotted for an example of predetermined composition, are fairly characteristic and show approximately the same appearance for the other compositions according to the invention; in particular, it will be seen that the acceptable value of Rml is obtained for a fairly narrowl zone of the eld of work; inthe present case: RID-07 for 3.2 oersteds Hm 3.6 oerstcds.

The influence of the temperature will be shown with reference to FIGS. 9, and 11.

In these three figures the curves C5, C, and C9 relate Y for two hours in an oxygen atmosphere.

FIG. 9 represents the variation of themaximum induction Bm, marked at Hm=10 oersteds between -40 C.

and j-l-lOO" C.; in considering the range from 20 C. to 80 C., it is possible to deduce therefrom:

aB=-0.99 for the compound 48Fe203, 47NiO, 5CuO aB='-01.l3 for the compound 49Fe203, 36Ni01, 10Zn0, 45CuO.

FIG. 10 represents the variation of the coercive eld Hen, for a cycle marked at Hm-f-lO-oersteds between -40 C. and +100" C.; in considering the range from' 20 C. to 80 C., it `is possible to deduce therefrom:

Y aHem=-0Ll2 for the compound 49Fe2O3, 36Ni0, 10ZnO, 5CuO.

Grinding-The mixture of oxides is Vground inpatl iron mill with steel balls, usually for 1.2-48 hours, with a weightk of distilled water which is substantially `double the weight of the mixture of oxides. Y Y

Pressing-The intuence of the stampingpressure "is important. This must be fairly high, so that the'moment at saturation of the finished product may be suiciently high, and on the other hand, fairly low so that'the shrinkvage during sintering may be great.V f 1 A pressure of about 5 tons per cm?, corresponding ,to

linear shrinkages of about 15%, has given good resultat@ it is possible to go from `0.5 to 15 tons per om?.

Heat treatment-In order to obtain the optimum properties, the temperature of annealing must be experimentally adjusted for each compositionrgenerally speakj ing, the more copper is contained inthe ferrite, the more it is necessary to reduce the temperature ofA annealing;

the atmosphere of annealing will be pure oxygen.

EXAMPLES FIGS. 12, 13, 14, 15 and 116` represent the.` hysteresis i cycles, plotted in direct current, for a maximum eld of 10 oersteds and for another `smaller iield if necessary.' In the latter case, the figure comprises two cycles, that 1 plotted for a eld lower than 10 oersteds being inside the 1 other.

ball mill, Vwith a capacity of 16 litres, containing about 3 kg. of mixture, about 6 litres of water and 2O kg. of balls. n

The annealing is carried out at 1,200 C. for 2 hours in oxygen.

The properties found are summarised in the following y FIG. 1,1 represents the variation of the coefficient of Table I- 1 v i 'TABLE I Molecular Composition in percent Magnetic Characteristics Y Fig. Nos. Y

F0205 A1203 Nio ouo zno Hm, Bm. Hom., am Rm Pi, Pv F.. Oe gauss 0e. Y

11 11 1 11 1 1 s is s 1111 13 49 0 36 5 1 {3.4 2:2140 22 0.?05 "'i s0 41000 0-2 11 -1 11 1 11 1 11 11 at a as .1.3 as 1111 0 15 49 0 31 5 15 i 3 2,770 1.3 Y 0.90 150 5,000 0102 16 475 0 32.5 15 5 1o 2,630 2.0 0.87 s0 5,000 `1.0

the rectangular m, for a cycle marked at Hm=10 oersteds, between -40 C. and +100 C.; in considering the range from 20 C. to 80 C., itis possible to deduce therefrom am=-0.02 for the compound 48Fe203, 47NiO, 5CuO. am=0-09 for the compound 49Fe203, 36NO, lOZnO, 5CuO.

It will thus be seen that these materials have coefv cients of variation, as al function of the temperature, which are remarkably low in relation to those normally obtained on other types of ferrites, although the addition of a certain percentage of ZnO has the elect of increasing these coefficients of variation. V

METHOD OF MANUFACTURE l Composition dnd Nature of the Oxides Employed Forv these mixtures a ferricoxide, Fe203, copper oxide, CuO, nickel oxide, NiO are employed 4and,.if necessary,

` zinc oxideZnO, aluminum oxide A1203 and chromic oxide which is a Vcombination `with vlow viscosity,V of nonv The content of each of these impurities must be less than 10.05 by weight.

METHODS OF APPLICATION OF THE STRESSES Magnetic materials having theV compositions indicated above and subjected to the grinding', pressing and heat treatment described, are then subjected to stresses by one ofthe known methods given in a non-limitative Vmanner, previously. As an example, among others, in order to obtain the compression stresses, use is made of coating the core with a thermosetting resin which permits a considerable shrinkage (10 to 15% after polymerisation) and does not split. 1

As a non-limitative example, we use: .Y (1) A ycasting resin which hardens when cold and saturated polyester resins', `with `monostyrene which can be solidified by polymeri'sation, with release of heat and considerable' shrinkage in the uncharged` state. v

(2) A cobalt accelerator; in principle: cobalt naph thenate or octoate dissolved in a solventsuch as those" commercially known as white spirit or styrolene.

(3) A catalyst `of benzoyl peroxide paste.

Ratio of proportions: u s g 1 50 Resin Catalyst g-- p 5 Accelerator drops.. 2

The advantages obtained by applying these arel ex,`

The grinding is carried out for 48 hour-s with an iron i plained hereinafter for the particular case of coating of the magnetic material by polyester resin.

FIGURE 17 shows a core of magnetic material 1 comf pletely coated in polyester resin 2, in which a hole 3 which T is chosen from the group consisting of aluminum and chromium with the values expressed in molecular percentages, x-i-y is between 47 and 50, y is between 0 and 2, v is between 3 and 15, u-I-v is between 35 and 53 has been arranged to avoid the splitting of the resin in 5 and s is between 0 and 15, x, y, u, v and s being equal to shrinkage. FIGURE 18 represents the same core 1 ex- 100, pressing said mixture into core form at a pressure ternally banded by polyester resin 4. between .05 and tons per square centimeter and heat- Instead of coating each core individually, it is possible ing the pressed core at a temperature between 900 C. and to make a matrix pro forma by coating a perforated plate 1300 C. in oxygen followed by cooling for a period of or by coating -a set of cores regularly Iarranged in a plate 10 15 hours. with polyester resin, 2. A ferrite having a substantially rectangular hysteresis EXAMPLE cycle and a negative coeicient of magnetostriction and FIGURE 19 shows thehysteresis cycles, for diierent mad? from a stamng mammal having a composition ac" values of the ield Hm, of a core of magnetic material in 15 cordmg to the formula' which the starting composition in molecular percentage is: Y F O T O N-O C O Z 4aFe203, 47Ni0, sono. x e2 3y 2 3 1 v u s no The conditions of sultermg are: hours at 1,200 C., in which T is chosen from the group consisting of alumill OXYgeD- h num and chromium, x-l-y is between 47 and 50, y is` be- In the whole of the followmg, the characterrstlcs of the 2O tween 0 and 2, v is between 3 and 15 +V is between 35 hystresls cycle have be@ maled on. the uxmer on and 53, and s is between O and 15, all expressed in molectOfOldl CONS 0f aPPfOXlmtae dlmellslons ular percentages, the sum of x, y, u, v and s being equal External diameter mm 34,7 to 100, the material being subjected to a pressure of be- Internal diameter mm 27.2 tween .05 and 15 tons per square centimeter and heated Height mm 3 8 25 -to a temperature between 900 and 1300 C. in an oxygen atmosphere followed by slow cooling. Woud wh (dtffm Copprzgg of OB) mm"d1ain` 3. A core of ferromagnetic material as defined in claim ffe) rrtml dieamegnfginducg wg; me 2, and means to exert artificial mechanical pressure on The cycles a, b and c correspond to the uncoated core Sald core' antd cyclfs d e g and h t0 the com coated Wlth Poly' 30 References Cited in the le of this patent es er resin.

The characteristics obtained are given in the following UNITED STATES PATENTS table: 2,532,876 Asche et al. Dec. 5, 1950 Core Curves t., C. Hm, Bm, Bm, Hm, m Rm Km Ph P' Oe. Gs. Gs Oe.

a 20 10 1. 540 1.200 4.0 0. 78 110 470 Not subjected to external b 20 5 537 454 3.1 0.80 0.30 3.5 60 310 mechanical stresses c 20 2 40 20 d 23 10 2. 580 2. 540 2.4 0. 98 n e 23 5 2. 500 2. 450 2. 4 o. 98 o Coated with polyester resin i f 23 3 2.260 2.240 2.4 0.98 0.98 150 a7 g 23 2.7 1.200 1.180 2.4 0. 98 0. 92 30 gv h 23 2.4 0 0 The response time r, for the example under considera- 2,551,711 Snoek et al. May 8, 1951 tion and with the definition and the dimensions previously 2,565,861 Leverenz et al. Aug. 28, 1951 given, is comprised between 1.0 and 1.5 ms. 2,579,267 Leverenz et al. Dec. 18, 1951 2,601,212 Polydoroif June 12, 1952 Values of the respectlve lntensitles. 2,685,568 Wilson Aug. 3 1954 Complete pulses ma 500 2,723,239 Harvey Nov. 8, 1955 Partial pulses ma 250 2,744,873 Piekarski May 8, 1956 For an uncoated Core: Bruner May 29, Useful signal (response given by the core after the 2849312 Pfterman Aug 26 1958 Second pulse), mv f 55 2,962,445 Plerrot et al. Nov. 29, 1960 1 nterlfegeng signal (response given by the core after the FOREIGN PATENTS as pu se mv.

Ratio useful signal/interference signal, 2.1. great infin? Aug- 17 1955 Response time; between 1 and 1.5 ms. 752659 Gigi E obciza coated core, according to FIGURE 17 or 18, we 60 1:10,]:654 France Aug. 10; 1955 Useful signal, mv 1,122,258 France May 22, 1956 ISligtnal intrierencf/"gnh 3 5 OTHER REFERENCES a 1o use u -s1 n 1n e erence, Response time? between 1 and 1.5 las. for the same 65 Gorter: Phihps Research Reports, vol. 9, No. 6, p. 432, relative values of current intensity. December 19541 Wha,t We Claim is: Kordes et al.. Chenucal Abstracts, vol. 46, col. 4411, 1. Method of manufacturing a unit comprising a fer- M3135 25th1 95F2' rite body having a negative coeicient of magnetostric- 5950721); ermmagnetlsm Van Nostrand 1951 Pp' tion and a substantially rectangular hysteresis cycle Which 70 comprises preparing a homogeneous mixture of oxides according to the formula xFe2O3yT2O3uNiOvCuOsZnO in I. of the Institute of Electrical Engineers, Japan, November 1937, page 5, June 1939, pp. 276, 279. 

2. A FERRITE HAVING A SUBSTANTIALLY RECTANGULAR HYSTERESIS CYCLE AND A NEGATIVE COEFFICIENT OF MAGNETOSTRICTION AND MADE FROM A STARTING MATERIAL HAVING A COMPOSITION ACCORDING TO THE FORMULA: 